In battery applications, porous sheets are used as separators. Battery separators are porous sheets that are interposed between an anode and cathode in a fluid electrolyte. For example, in lithium ion batteries, lithium ions (Li+) move from the anode to the cathode during discharge. The battery separator acts to prevent physical contact between the electrodes while allowing ions to be transported. Typical prior art separators include microporous membranes and mats made from nonwoven cloth. Battery separators are ideally inert to the electrochemical reactions that occur in batteries. Therefore, various polymers have been used to form battery separators.
In the case of fuel cells, porous sheets are used as gas diffusion layers and supports for polyelectrolyte membranes. Gas diffusion layers play a multifunctional role in proton exchange membrane fuel cells. For example, gas diffusion layers act as diffusers for reactant gases traveling to the anode and the cathode layers while transporting product water to the flow field. Gas diffusion layers also conduct electrons and transfer heat generated at the membrane electrode assembly to the coolant, and act as a buffer layer between the soft membrane electrode assembly and the stiff bipolar plates. Although the present technologies for making gas diffusion layers for fuel cell applications work reasonably well, improvement in properties and cost are still desirable.
Accordingly, there is a need for improved methods for forming fuel cell membranes, diffusion media, fuel cell electrodes and battery separators.